Optical networks are continuously evolving in order to meet ever-increasing voice and data traffic demands. There is a need for increased multi-channel capacity in wavelength division multiplexed (WDM) and dense wavelength division multiplexed (DWDM) systems, increased reliability, and reduced cost. Conventional optical networks incorporate forward error correction (FEC) in order to improve system margins and protection switching in order to improve system reliability.
FEC is a methodology whereby bits that have been improperly received, or not received at all, are detected and corrected, or replaced. Each frame of working data includes corresponding error-check data. Among other things, FEC allows a system to utilize a decreased signal-to-noise ratio (SNR), while maintaining a fixed error probability. Advantageously, a decreased SNR means that optical amplifiers may be spaced at longer distances, reducing cost, and that a faster signal may be used, for example. Disadvantageously, the inclusion of error-check data means that increased bandwidth, as well as increased transmitter and receiver complexity, are required.
Protection switching is a methodology whereby alternate optical paths, links, or channels are selected and used in response to the failure of a given optical path, link, or channel. Typically, such failures include direct failures that are detected on the receive side of the system. Protection switching is incorporated into a variety of transmission protocol standards, such as the synchronous optical network/synchronous digital hierarchy (SONET/SDH) transmission protocol standard, etc. For example, it may be required that voice or data traffic is restored within 50 ms of a direct failure.
Several conventional systems and methods use multiple working channels on a single optical link to protect against polarization mode dispersion (PMD), for example. PMD results as light travels down a single-mode fiber in two inherent polarization modes. When the core of the fiber is asymmetric, the light traveling along one polarization mode travels faster or slower than the light traveling along the other polarization mode, resulting in a pulse overlapping with others, or distorting the pulse to such a degree that it is undetectable by a receiver. PMD concerns are compounded in today's high-speed transmission optical networks. Further, PMD varies dynamically with temperature changes, infinitesimal asymmetries in the fiber core, etc., and impacts diverse wavelength channels differently. Thus, it is wavelength and time-dependent.
S. Särkimukka, A. Djupsjöbacka, A. Gavler, and G. Jacobsen (“Mitigation of Polarization-Mode Dispersion in Optical Multichannel Systems,” J. Lightwave Techn., Vol. 18, No. 10, October 2000, pp. 1374-1380) disclose an approach whereby signal quality is monitored for PMD degradation and, if detected, protection switching between a failed channel and a working channel is performed. However, in the proposed system, an optical switch is disposed between the optical transmitters and an optical multiplexer (MUX). Because individual data input signals are connected to the optical transmitters, this configuration does not provide a workable solution for redirecting a specific data input signal from a failed optical transmitter to a working one. In a practical implementation, a switch must be disposed between the client input connection and the optical transmitter.
D. Penninckx, S. Lanne, and H. Bülow (“WDM Redundancy to Counteract PMD Effects in Optical Systems,” European Conf. on Optical Comm., 2001) disclose an approach whereby all of the client signals are encoded with FEC, and all of the individual client data streams are bit-wise spread across all of the available optical channels in the system. On the receive end, the data streams from the optical channels are detected, re-synchronized, and bit-wise reassembled, and individual client data streams are recovered through FEC. Thus, if a few optical channels fail, these failed optical channels would affect a small fraction of the bits for all of the data signals equally. FEC allows for the recovery of the original data. This approach, however, suffers from requirements associated with complicated bit spreading and bit re-synchronization electronics. Typical FEC codes can correct on the order of 1 or fewer bits per 1000. This means that a transmission system would require 1000 channels, only 1 of which is allowed to fail at a time, which is not practical.
Conventional multi-channel systems are guaranteed to perform error-free on all channels simultaneously. As a result, such systems are limited by the performance of a worst-case channel. In optical communications, channel impairments include: amplified spontaneous emission (ASE) noise common to all erbium-doped fiber amplifiers (EDFAs) and contributing to the noise figure of the EDFAs, causing the loss of SNR; non-linear impairments, such as self-phase modulation (SPM) caused by the non-linear index of refraction of glass and its variance with optical power level, causing a frequency chirp that interacts with a fiber's dispersion to broaden a pulse, cross-phase modulation (XPM) also caused by the non-linear index of refraction of glass and its variance with optical power level, causing signals to interact, four wave mixing (FWM), common in DWDM systems, where multiple wavelengths mix together to form new wavelengths, or interfering products, that fall on the original wavelengths, becoming mixed with the signals, distorting the signals and causing degradation, etc.; chromatic dispersion (CD) caused by the fact that different colored pulses of light, with different wavelengths, travel at different speeds, even within the same mode, being the sum of material dispersion and waveguide dispersion; PMD, described above; polarization-dependent gain/loss (PDG/PDL); and the like. Further, individual channel optical transceiver (XCVR) and MUX/DEMUX performance may vary due to manufacturing tolerances and aging. Such impairments do not affect all channels equally, but, rather, present a finite statistical distribution. Thus, it is not possible to predict an individual channel performance, and system performance calculations are typically performed using worst-case scenario approximations.
Further, many of the channel impairments described above, and others, are time-dependent. For example, amplifier ripple may depend on loading, temperature, etc. PMD may depend on fiber plant conditions and polarization state. XPM and FWM may depend on polarization state of a channel and its neighbors, as well as fluctuating channel powers. The tolerance window for many of these impairments decreases significantly with increasing data rate (i.e. 40 Gb/s and above).
Disadvantageously, current protocols (i.e. SONET/SDH) protection switch on failed signals. This requires the pre-provisioning of bandwidth through an alternate physical path, which is typically implemented in a ring-like structure for SONET/SDH protection.
Thus, what are still needed in the art are multi-channel protection switching systems and methods for increased reliability and reduced cost. Preferably, the multi-channel protection switching systems and methods use pre-FEC rate measurements, and changes in pre-FEC rate measurements, to detect and avoid potential link failures before they occur. Preferably, the multi-channel protection switching systems and methods also use “wavelength-hopping” and other protection schemes to alleviate wavelength and time-dependent optical propagation impairments.